Powdered filtration media are capable of separating fine particles from a wide variety of liquids and are also used in selected air filtration processes. Materials used for powdered filtration media include processed forms of biogenic silica, including diatomite and rice hull ash, diatomite, expanded perlite, expanded volcanic ash, expanded pumice, and cellulose. A wide variety of filters utilize powdered filtration media, such as fixed bed filters and vacuum or pressure filters. Further, powdered filtration media can serve as a pre-coat or body-feed for pressure filters or a pre-coat in rotary vacuum precoat filtration (drum filters). While the exact removal of particles suspended in a liquid can vary based on the liquid, the processing conditions and the particles, in general, it is believed that diatomite may remove particles as small as about 0.5 microns from liquids, while expanded perlite is better suited for coarser particles of about 5 microns or larger.
Diatomite consists of the skeletal remains of certain algae, generally referred to as diatom frustules. Diatom frustules consist of relatively pure (98-99.5 wt %) amorphous and hydrated silica in form of opal-A. In addition to diatom frustules, diatomite ore generally incorporates other minerals and rocks, such as clays, feldspars, quartz, volcanic ash and other impurities. Besides free moisture, bound water and organic contaminants, diatomite ore usually contains about 80-95 wt % amorphous silica and about 5-20 wt % other minerals. As a result, diatomite raw materials that are either used as natural products or as feed for further processing contain a mineral system, not just the pure diatom frustules, and the existence of this mineral system places constraints on many of the properties, such as extractable chemistry, density, permeability and mineralogy, including crystalline silica content.
After it is mined, crushed and dried, diatomite ore may be classified to remove some of the mineral impurities. U.S. Pat. Nos. 5,656,568 and 6,653,255 teach a method for making high purity diatomite products by wet beneficiation and acid wash. While the removal of some non-diatom-derived minerals can improve some product properties, such as extractable chemistry and density, it can also have a negative impact on some other properties, such as permeability, as some of the other minerals often found with diatomite can aid in the agglomeration of particles during calcination and flux calcination.
Calcination and flux-calcination, the common terms used to describe processes used to agglomerate the particles contained in diatomite ore, increase the average particle size, porosity and permeability of the products. In both calcination processes, diatomite powder is commonly heated in a rotary kiln.
Flux-calcination is similar to straight calcination, but includes the addition of a fluxing agent to the diatomite powder, usually soda ash, before the calcining step. Adding a fluxing agent further promotes sintering of the diatomite particles and increases the average particle size, porosity and the permeability beyond that achieved by straight calcination.
While other fluxes have been used in small quantities over the years, sodium-based fluxes, including soda ash (sodium carbonate) and common salt (sodium chloride) have been the most popular fluxes. Various other fluxes are described in the prior art, including other alkali metal fluxes. For the purposes of this disclosure, we include the following metals as alkali metals: lithium, sodium, potassium, rubidium, cesium and francium.
In summary, calcined diatomite is more permeable and has a larger average particle size than un-calcined or natural diatomite, and flux-calcined diatomite is more permeable and has a larger average particle size than both calcined and natural diatomite.
Perlite is a volcanic rock of alumino-silicate glass formed through the rapid cooling of magma or lava of a rhyolitic composition, followed by hydration. Perlite typically contains about 2-5 wt % bound water that causes the perlite to expand or “pop” when the perlite is rapidly heated to a temperature range of 1600-2000° F. (871-1093° C.). During expansion, the water contained in perlite turns to steam and, as the perlite softens, it expands rapidly. Perlite is thermally expanded to make low density, powdered products for use in a variety of applications, including liquid filtration. For filtration applications, expanded perlite is milled at various levels of intensity and is then classified to produce powdered filtration media products of different particle size distributions and permeabilities. Volcanic ash, tuff, obsidian and pumice are also hydrated natural glasses of volcanic origin possessing compositions similar to that of perlite. For the purposes of this disclosure, the term “perlite” includes all types of hydrated natural volcanic glasses.
Upon expansion, perlite may form enclosed spheres with little or no open pore structure. These enclosed spheres are referred to as “floaters.” While floaters may be useful for some applications, such as in insulation or horticulture, when used as powdered filtration media, floaters are buoyant, may not actively filter small particles and can damage certain types of filtration equipment. As a result, producers of powdered perlite filtration media mill the expanded perlite to open the pore structures of the particles thereby reducing the floater content. The coarser or more permeable grades of expanded perlite normally contain more floaters than the finer or less permeable grades of expanded perlite.
Common crystalline silica minerals include quartz, tridymite and cristobalite, and less common or rarer are melanophlogite, moganite, keatite, coesite, stichovite and seiferite. The presence of crystalline silica in a powdered product can be concerning because small crystalline silica particles can be inhaled, and prolonged exposure to respirable crystalline silica particles may lead to undesirable health effects. As a result, there is a need for powdered filtration media products that possess the desirable filtration characteristics of diatomite but which contain reduced or non-detectable levels of crystalline silica.
Current regulations require that a product sold in the United States without a warning label must contain less than 0.1 wt % total crystalline silica, and in Europe, any product without a warning label must contain less than 1 wt % respirable crystalline silica. Particles finer than 10 micrometers are considered respirable. The respirability describes the probability of a particle of a certain size being inhaled into a human lung. The respirability of a particle increases with its decreasing size and is described by a respirability function. For example, particles of 10 micrometers and larger have a respirability of 0%, those of 1 micrometer have a respirability of 94.8% and for those of 0.1 micrometer have a respirability of 99.5%. Content of respirable particles in a powdered material is generally calculated based on the content and particles size distribution of such particles finer than 10 micrometers. The content of respirable crystalline silica in a sub-10-micrometer size fraction in a powder material commonly is computed based on three factors: the content of the particle size fraction in the powder material, respirability of the size fraction and the content of crystalline silica in the size fraction. The total respirable crystalline silica content of a powder material is then calculated by the summation of the contents of respirable crystalline silica in all sub-10-micrometer fractions.
Opals are amorphous hydrated silica minerals and are characterized by the existence of spherical nano-clusters of hydrated silica and adsorbed water. Naturally occurring common opaline minerals are opal-A, opal-CT and opal-C. Opal-A can form naturally from supersaturated aqueous solutions of silicic acid, but is more commonly formed in biological processes by various species of plants, such as diatoms, bamboo, rice plants and a number of other plant species. Partial dehydration and heating, either geologically or artificially, of opal-A brings short range ordering and transforms it to opal-CT or opal-C, depending on the degree of ordering and level of dehydration. Opaline silicas are non-crystalline and, at this time, the risks of inhaling opaline silica dusts have not been demonstrated to be harmful.
Most diatomite ores contain crystalline silica, in the form of quartz, and the removal of this quartz from these ores is sometimes difficult or impossible. During both calcination and flux-calcination of diatomite, a mineral phase change occurs in which some of the amorphous silica is dehydrated and converted from opal-A to opal-CT or opal-C and, in some cases to crystalline silica, most commonly in the form of cristobalite (and less commonly, quartz).
Opal-C is often formed during the calcining process, and until recently it has not been possible to distinguish opal-C from cristobalite in calcined or flux calcined diatomite. Lenz et al., in a recently filed
United States provisional patent application (Ser. No. 62/245,716) teaches a method to distinguish opals from cristobalite, and the present disclosure makes use of this teaching.
Opal-C may be converted to cristobalite with further heating, and this transformation of opal-C can be promoted by the addition of a fluxing agent, especially a fluxing agent containing sodium. A fluxing agent, when used in calcination, reduces the softening or melting temperature and viscosity of amorphous biogenic silica, and can result in the formation of increased levels of cristobalite when the silica cools after the flux calcining process is complete.
Without being bound by theory, it is believed that sodium fluxing agents may increase or enhance cristobalite formation during flux-calcination because of the small ionic radius of sodium, which allows sodium ions to fit into the lattice interstitials of the cristobalite crystalline structure. While other alkali metal salts, especially potassium salts, have been proposed for use as low cristobalite fluxing agents for diatomite agglomeration, there is still some formation of cristobalite when these agents are used. Furthermore, cristobalite formation during calcination of diatomite increases with the use of higher calcination temperatures and/or longer heating periods, as well as the use and dosage of a fluxing agent. As a result, straight-calcined and flux-calcined diatomite may contain cristobalite from very low levels up to as much as 80 wt % or more.
As discussed above, in many cases calcination of diatomite transforms, partially, the opaline silica from opal-A to opal-C. Further heating, and in many cases further heating assisted by a sodium fluxing agent, transforms opal-C to cristobalite. Due to the close proximity between primary peaks of opal-CT, opal-C and cristobalite x-ray diffraction (XRD) patterns, opal-CT and opal-C in diatomite products have been traditionally attributed to cristobalite. As mentioned above, a powder XRD scan method was recently developed by Lenz et al. to differentiate opal-C and opal-CT from cristobalite, and this technique has been used to differentiate cristobalite from opals in the present disclosure.
The diffraction pattern of cristobalite contains sharp Bragg's peaks, most notably at 22.02°. 36.17°, 31.50°, and 28.49° 2θ with Cu Kα x-ray (λ=1.54056 Å). The diffraction patterns of opal-C and opal-CT are less well-defined, with broader and fewer peaks that may be indicative of radial scattering and not true Bragg's peaks. All three share a similar primary peak near 22° 2θ which corresponds to a d-spacing approximate 4.0 Å according to the Bragg law and a secondary peak near 36° 2θ. But the peaks near 31.5° and 28.5° 2θ are very poorly developed for opal-C or are missing for opal-CT. In summary, the opal-C and opal-CT diffraction patterns differ from that of cristobalite in the following ways: a primary peak at a slightly lower 2θ angle or higher d-spacing than 4.03 Å for cristobalite, a broader primary peak as measured using the FWHM (full width at half maximum) statistic, lack of defined peaks at 31.5° and 28.49° 2θ, and a much more significant amorphous background. For example, Elzea and Rice (Clays and Clay Minerals, vol. 44, pp. 492-500, 1996) presents XRD patterns of 24 opal-C and opal-CT samples which have d-spacings ranging from 4.03 to 4.11 Å, between that of samples of cristobalite (4.03 Å) and tridymite (4.11 Å), and FWHM from 0.2 to 1.0° 2θ, well above that of about 0.15° 2θ for samples of cristobalite and <0.20 2θ for tridymite.
Perlite ores may also contain different levels of unexpandable minerals, such as rhyolite, feldspar and quartz. After expansion, the perlite may be processed to remove heavier, unexpanded mineral particles. Even with post-expansion separation processes, many expanded perlite products may still contain some crystalline silica in the form of quartz.
When a liquid passes through a powdered filtration media, soluble components of the filtration media may dissolve into the liquid and eventually remain in the filtered liquid. Users of powdered filtration media often set limits on the amount and type of soluble components that can pass into the filtered liquid. In addition, many countries establish acceptable levels of dissolved components in products that pass through powdered filtration media, such as food and pharmaceutical products. For example, in the United States, the Food Chemicals Codex (FCC) includes standards with regard to the purity and quality of certain products that involve food or beverage processing. In addition, industrial associations have set standard analytic methods for quality and purity. For example, beer is typically filtered using a powdered filtration media, such as a calcined or flux-calcined diatomite. Solubility of a substance from a powdered filter media varies with the extraction method (for example, the type of solvent and the conditions under which a powdered filtration medium is in contact with the solvent). The American Society of Brewing Chemists (ASBC) and the European Brewery Convention (EBC) have both set standard analytical methods for extractable or soluble content of iron from powdered filtration media.
The permeability of powdered filtration media is a measure of the rate at which a standard liquid can pass through a standard preparation of the media under standard conditions in the unit of “darcy.” A 1-darcy medium, when constructed to have a 1-cm2 surface area and a 1-cm thickness, will allow a 1-centipoise (1-mPa-s) viscosity liquid (e.g., water at 20° C.) to pass through at a rate of 1 milliliter (ml) per second under a differential pressure of 1 atmosphere (101,325 Pascal). Common powdered filtration media can have permeabilities ranging from less than 0.01 to about 20 darcy or more. For powdered filtration media, especially for products composed of diatomite, there is a reasonably predictable relationship between permeability and the ability of the media to remove particles from liquids, often referred to as particle size exclusion. Specifically, powdered filtration media with low permeabilities generally are able to remove finer particles from liquids than powdered filtration media with high permeabilities. Higher permeability powdered filter media usually are able to filter a liquid at a higher throughput but at the expense of a lower or worse filtrate clarity than lower permeability powdered filter media.
The wet bulk density of a powdered filtration medium reflects the void volume or porosity of a filter cake formed from a unit mass of the powdered filtration medium. If two powdered filtration media possess the same particle size exclusion capabilities, but one possesses a lower wet bulk density, the unit consumption of the lower density product, in terms of mass, will also be lower and therefore more cost effective (at equal product pricing). As mentioned above, powdered filtration media made from diatomite provide a greater size exclusion or can remove finer particles from liquids than powdered filtration media made from expanded perlite. Powdered diatomite does not generally contain floaters, as opposed to powdered expanded perlite filtration media, which usually contains floaters. On the other hand, powdered expanded perlite typically has a lower wet bulk density and is more likely to contain low or non-detectable levels of crystalline silica than powdered calcined diatomite. U.S. Pat. Nos. 6,464,770 and 6,712,898 teach methods of making perlite products with reduced floater content and controlled particle size distribution through mechanical classification. An improved filtration media product would ideally contain the best attributes of both diatomite and expanded perlite.
Indeed, composites of diatomite and expanded perlite are known for use as powdered filtration media. U.S. Pat. Nos. 6,524,489 and 5,776,353 teach methods of making composite filtration media from various components including diatomite and expanded perlite with or without a fluxing agent. However, these products have never been commercialized, perhaps due to one or more of the following reasons: the products produced without a fluxing agent have only a relatively low permeability; the products produced with a sodium-based fluxing agent were all produced under conditions that produced measurable levels of cristobalite; the ores used in the examples in the patents all contained approximately three to five percent quartz, which was not removed in the manufacturing process; at the time the prior art was developed, methods to distinguish opal-C and opal-CT from cristobalite did not exist; there is no indication that the inventors of the prior art were able to produce products that possess many of the key properties required for a filter aid to be acceptable in many applications, including soluble impurities and floater content; the most successful flux used in the prior art in suppressing cristobalite formation was boric acid, a very expensive material that substantially increases the soluble aluminum and calcium of the composite filtration media.
A specific formulation of the composites of the prior art was disclosed in a letter to the US Food and Drug Administration (FDA). In this letter, the product is described as containing a very low level of diatomite. Such a product would have little capability to remove fine particles from a liquid because it would behave more like perlite than like diatomite.
Some additional information on the composite prior art is included below. As mentioned above, permeabilities obtained by the prior art without a fluxing agent are too low (about 0.2 darcy) for many commercial applications (see Example 1 of '489 patent, col. 16, line 8). The only examples with sufficiently high permeabilities make use of a fluxing agent, either boric acid or soda ash, or comprise very low amounts of diatomite, which also limits their commercial feasibility (for particle size exclusion reasons). Boric acid and other boron-containing fluxes are expensive and they lead to an increase in soluble calcium and aluminum, which is unacceptable to many producers of filtered liquid products. A sodium-containing fluxing agent such as soda ash may lead to the formation of cristobalite. In addition, as shown in publically available material safety datasheet (Celite MSDS No. 2200, 2012), the diatomite feed material (Celite® 500) disclosed in these patents and used in most examples for making diatomite-perlite composite samples contained up to 4% quartz. Its use in the composite examples disclosed in the patents in a range of 50-90% contributes up to 2 to 4% quartz in the composite products. Furthermore, Example 9 of patent '353 uses a highly flux-calcined diatomite Celite® 560 which contains up to 50% cristobalite (Celite MSDS No. 2410, 2009) in a feed blend with perlite in the 90/10 ratio and the feed blend was calcined with additional 5 wt % soda ash at 1500° F. (816° C.). Thus, one of ordinary skill in the art, understands that the produced composite of patent '353 contains a high level of cristobalite though such high level of cristobalite is not explicitly disclosed in the patent. Further, a publicly disclosed composite product made under these patents included less than 5 wt % diatomite and with sodium flux-calcination, presumably to keep both quartz and cristobalite contents sufficiently low. Composite products containing low levels of diatomite behave, from a particle size exclusion standpoint very much like perlite, in other words, they are not able to remove fine suspended particles from liquids.
As a result, there is a need for an economical, powdered composite filtration media of diatomite and expanded perlite having a wide permeability range, containing very low or non-detectable crystalline silica, low floater content, and low soluble metal content, which can be produced at an attractive cost.